In recent years, for semiconductor devices, a tendency toward higher performance and reduction in thickness and size of electronic equipment have resulted in an ever-increasing demand for higher integration density and higher functions such as typified by ASIC (application-specific IC) of LSI.
In the semiconductor device having increased integration density and function, inductance within the package has become unnegligible for high-speed processing of signals. In order to reduce the inductance within the package, the number of connecting terminals of the power source and ground has been increased to lower the substantial inductance.
The higher integration density and higher function of the semiconductor device has resulted in increased total number of external terminals (pins) and an ever-increasing demand for an increase in number of terminals (pins).
In multi-terminal (pin) IC, particularly semiconductor devices, such as ASIC typified by gate array and standard cell, microprocessor unit, and DSP (digital signal processor), those using a lead frame include surface mounting type packages, such as QFP (quad flat package). In QFPs, the number of pins up to 300 pins has been put to practical use.
In QFPs, a single-layer lead frame 1310 as shown in FIG. 44b is used. As shown in FIG. 44a (cross-sectional view), a semiconductor element 1320 is mounted on a die pad 1311, an inner lead in its front end 1312A subjected to treatment, such as silver plating or gold plating, is connected to a terminal (an electrode pad) 1321 of the semiconductor element 1320 through a wire 1330, plastic molding is performed using a resin 1340, a dam bar section is cut, and an outer lead 1313 section is bent in a gull-wing form. In such QFP, the structure is such that an outer lead for electrical connection to an external circuit is provided in four directions of the package. This structure can cope with a demand for an increase in the number of terminals (pins). The single-layer lead frame 1310 used herein is generally prepared by fabricating a metal sheet having excellent electrical conductivity and high strength, such as Kovar, 42 alloy (42% Ni-iron), or copper-base alloy, by etching using photoetching, stamping or the like into a lead frame as shown in FIG. 44b.
A demand for an increase in signal processing speed and an increase in performance (function) of a semiconductor element in recent years, however, requires a further increase in number of terminals.
By contrast, in QFP, a reduction in external terminal pitch permits QFP to cope with a demand for a further increase in number of terminals. In the reduction in the external terminal pitch, however, the width of the external terminal per se should also be reduced, resulting in lowered external terminal strength. This unfavorably poses a problem associated with positional accuracy or flatness accuracy in molding of the terminal (formation of gull-wing). Further, in QFP, the step of mounting in a reduction in pitch becomes difficult with reducing the pitch of the outer lead to 0.4 mm, 0.3 mm or a smaller pitch, which necessitates realization of a highly advanced board mounting technique.
In order to avoid problems of mounting efficiency and mountability involved in the conventional QFP package, a plastic package semiconductor device called BGA (ball grid array), which is a surface mounting type package with the external terminal of the package being replaced by a solder ball, has been developed.
BGA is a generic name for a surface mounting type semiconductor device (plastic package) wherein the external terminal is constituted by solder balls which are arranged in a matrix form (an array form) on the back surface.
In general, in this BGA, in order to increase the number of input and output terminals, a semiconductor element is mounted on one side of the double-sided wiring board, an external terminal electrode for mounting a spherical solder thereon is provided on the other side, and the semiconductor element is electrically connected to the external terminal electrode through a through hole. Disposing solder spheres in array can widen the terminal pitch spacing as compared with the spacing in the semiconductor device using the conventional lead frame. As a result, the BGA can cope with a demand for an increase in number of input and output terminals without making it difficult to perform the step of mounting the semiconductor device.
BGA generally has a structure as shown in FIG. 39a. FIG. 39b is a diagram as viewed from the back surface (substrate) side in FIG. 39a. FIG. 39c is a diagram showing a through hole 850 section. This BGA comprises: a substrate 802 constituted by a heat-resistant flat sheet (resin sheet) typified by a BT resin (a bismaleimide resin); provided on one side of the substrate 802, a die pad 805 for mounting a semiconductor element 801 thereon and a bonding pad 810 for electrical connection from the semiconductor element 801 through a bonding wire 808; and, provided on the other side of the substrate 802, an external connecting terminal 806, constituted by a solder ball arranged in a lattice form or a zigzag form, for electrical and physical connection between the external circuit and the semiconductor device, the external connecting terminal 806 and the bonding pad 810 being electrically connected to each other through wiring 804, a through hole 850, and wiring 804A.
This BGA, however, has such a complicate structure that a circuit for connecting a semiconductor element mounted to a wire and an external terminal electrode for mounting a printed board after the formation of a semiconductor device are provided on both sides of the substrate 802 and these are electrically connected through a through hole 850. For this reason, influence of the thermal expansion of the resin often creates breaking in the through hole 850, posing a problem of reliability in the production.
In order to simplify the production process and to avoid the lowering of the reliability, various proposals have recently been made on PBGA (plastic ball grid array), wherein a circuit is provided using a lead frame as the core material, in addition to the structure shown in FIG. 39a.
The PBGA package using the lead frame generally has a structure, as shown in FIG. 40a, such that a lead frame 910 in its entirety is fixed onto an insulating, fixing film 960 with a predetermined hole formed in a place corresponding to an external terminal section 914 of a lead frame 910 and plastic molding is performed, or a structure, as shown in FIG. 40b, such that an inner lead is fixed with a fixing tape 960A.
The lead frame 910 used herein is constructed so that both the external terminal section 913 and the inner lead 912 have a thickness equal to the thickness of the lead frame material. After the fabrication of the external shape by etching, as shown in FIG. 41a, a connecting section 917, which extends to the front end of the inner lead 912 and is integrally connected to the inner lead to fix the inner leads to one another, is provided, and a supporting lead 915 for supporting the external terminal section is connected to a dam bar (a frame section) 914.
In the case of a semiconductor device 900 shown in FIG. 40a, as shown in FIG. 41, a lead frame (FIG. 41a) in its entirety is fixed using a fixing film 960 (FIG. 41b), the connecting section 917, for connecting inner leads to one another, which is primarily unnecessary, is removed by pressing to prepare a lead frame member 970, as shown in FIG. 41c, comprising a lead frame 910 and a fixing film 960. Numeral 920 designates an opening. In this case, use of an expensive mold is necessary for the production of the lead frame member 970, and the productivity is also low.
On the other hand, in the case of a semiconductor device 900A shown in FIG. 40b, a part, including the inner lead, of the lead frame rather than the whole lead frame is fixed with a fixing tape 960A, and a connecting section (not shown) for connecting inner leads to one another is then removed to prepare a lead frame member 970A comprising a lead frame 910 and a fixing tape 960A. Also in this case, use of an expensive mold is necessary for the production of the lead frame member 970A, and the productivity is also low.
Further, in the case of using the lead frame member 970 shown in FIG. 41c and in the case of using the lead frame member 970A with a part of the lead frame fixed (FIG. 40b), in the production of a semiconductor device, as shown in FIG. 42, the dam bar (frame section) 914 should be removed after plastic molding to separate the supporting leads 915 which have supported the external terminal section. In this case, the frame section is cut and removed by means of the mold, necessitating use of an expensive mold. In addition, the productivity is also low.
In the BGA, plastic molded type semiconductor device using a lead frame as the core material, as compared with a semiconductor device using the single-layer lead frame shown in FIG. 44b, when the number of terminals is the same, the pitch of the external terminals for connection to an external circuit can be widened, permitting the semiconductor to cope with a demand for an increase in the number of input and output terminals without rendering the step of mounting of the semiconductor device difficult. A reduction in pitch of the inner lead, however, is indispensable and has been required for a further increase in the number of terminals.
In order to cope with this, an etching method has been proposed wherein the inner lead section is formed in a smaller thickness than the lead frame material to achieve a narrow pitch.
One example of this type of the etching method will be described with reference to FIG. 43.
For simplification, the production of a lead frame, wherein only the inner lead has a smaller thickness than the lead frame material made of a copper alloy, will be described.
FIG. 43 is a cross-sectional view showing the front end of an inner lead in each step of forming an inner lead having a small thickness.
With respect to the place which should be subjected to external shaping to a thickness equal to the thickness of the lead frame material, a resist pattern having substantially the same shape and size is formed on both sides of the lead frame material, followed by etching.
In FIG. 43, numeral 1210 designates a lead frame material, numeral 1210A a small thickness section, numerals 1220A and 1220B each a resist pattern, numeral 1230 a first opening, numeral 1240 a second opening, numeral 1250 a first recessed section, numeral 1260 a second recessed section, numeral 1270 a flat face, numeral 1280 an etching resistant layer (a filler layer), and numeral 1290 an inner lead.
At the outset, both sides of a lead frame material constituted by a 0.15 mm-thick strip are subjected to cleaning, degreasing or the like, a resist composed of a mixed solution of an aqueous casein solution using potassium dichromate as a sensitizing agent is coated on both sides thereof, the resist is dried, and a predetermined area of the resist on both sides of the lead frame material using a plate having a predetermined pattern is exposed, followed by development to form resist patterns 1220A and 1220B respectively with a first opening 1230 and a second opening 1240 having respective predetermined shapes (FIG. 43a).
The first opening 1230 is provided for attacking the lead frame material 1210 in a solid form in the subsequent etching through this opening to a thickness smaller than the lead frame material 1210, and the second opening 1240 of the resist is provided for defining the shape of the front end of the inner lead.
Subsequently, both sides of the lead frame material 1210 with a resist pattern formed thereon are etched with a ferric chloride solution having a solution temperature of 50.degree. C. and a specific gravity of 46 Baume degree at a spray pressure of 3.0 kg/cm.sup.2, and the etching is stopped when the depth h of the first recessed section 1250 attacked in a solid form (a flat form) has reached a predetermined value (FIG. 43b).
Shortening the etching time in the second etching described below is the reason why the first etching is simultaneously performed from both sides of the lead frame material 1210. Simultaneous etching from both the sides in the first etching permits the total etching time of the first etching and the second etching to be shorter than that in the case of one side etching from the resist pattern 1220B side alone.
A resin resistant to etching is then coated as an etching resistant layer 1280 by die coating on the first recessed section 1250 attacked on the first opening 1230 side to fill up the first recessed section 1250 attacked in a solid form (a flat form). Further, the etching resistant layer 1280 is coated also onto the resist pattern 1220B (FIG. 43c).
The etching resistant layer 1280 is not always required to be coated on the whole surface of the resist pattern 1220B. Since, however, it is difficult to coat the etching resistant layer 1280 onto only a part including the first recessed section 1250, as shown in FIG. 43c, the etching resistant layer 1280 is coated onto the whole surface of the first opening 1230 side together with the first recessed section 1250.
Preferably, the resin constituting the etching resistant layer 1280 is fundamentally resistant to the etching solution and has flexibility to some extent at the time of etching. It may be of UV curable type.
When the first recessed section 1250 attacked on the side with a pattern for forming the inner lead front end being provided thereon is filled up with the etching resistant layer 1280, an advantage can be offered that the first recessed section 1250 is not attacked and does not become large during the etching in the subsequent step, the mechanical strength against high definition etching can be increased, and, hence, the spray pressure can be increased (3.0 kg/cm.sup.2) facilitating the progression of the etching in the direction of the depth.
Thereafter, the second etching is performed. In this case, the lead frame material 1210 is etched from the second recessed section 1260 side which is opposite to the side with the first recessed section 1250 attacked in a solid form (a flat form). By this etching, a through hole is formed to provide the small thickness section of the front end of the inner lead 1290 (FIG. 43d).
The face, parallel to the lead frame face, created by the first etching, is flat. On the other hand, two faces between which this face is sandwiched are recessed toward the inner lead side.
The second etching is followed by washing, removal of the etching resistant layer 1280, and removal of the resist film (resist patterns 1220A and 1220B) to prepare a lead frame wherein an inner lead 1290 has been finely fabricated (FIG. 43e).
The etching resistant layer 1280 and the resist layer (resist patterns 1220A and 1220B) are dissolved and removed with an aqueous sodium hydroxide solution.
The above etching which is performed in two separate stages is generally called two-step etching and is excellent particularly in accuracy.
In the production of a lead frame shown in FIG. 43, use of the two-step etching and devising of the shape of the pattern permit the external shape to be formed while partially reducing the thickness of the lead frame material.
The method for forming a thin inner lead in the lead frame is not limited to the above etching method.
Fine fabrication by the above method wherein the inner lead is formed in a small thickness is influenced by the shape of the second recessed section 1260 and the thickness t of the front end section of the final inner lead. For example, when the thickness t is reduced to 50 .mu.m, fine fabrication can be performed to such an extent that as shown in FIG. 43e, when the flat section width W1 is 100 .mu.m, the inner lead front end section pitch p is up to 0.15 mm. When the thickness t is reduced to about 30 .mu.m with the flat section width W1 being about 70 .mu.m, fine fabrication can be made to such an extent that the inner lead front end pitch p is up to about 0.12 mm. A further narrow inner lead front end section pitch becomes possible depending upon the thickness t and the flat section width W1.
In the lead frame prepared by the steps and the like shown in FIG. 43, the inner lead section becomes unstable with reducing the thickness of the inner lead, and, as shown in FIG. 44, the connecting section 917 for connecting the inner lead front end sections to one another should be removed. Further, as shown in FIG. 42, the dam bar (frame section) 914 should be cut and removed. This poses problems of productivity and cost, and, in addition, it is difficult to maintain the positional accuracy and quality of the inner lead. Therefore, solution to the above problems has been desired in the art.
On the other hand, in the plastic molded type semiconductor device, a reduction in package mainly by a reduction in thickness through the development of TSOP (thin small outline package) has been conducted. However, in small packages, such as TSOP, drawing of the lead, pin pitch, and adoption of multi-pin have reached the limit.
Under these circumstances, a semiconductor device, which, as compared with TSOP and the like, is smaller in size and lower in cost and has better mountability, and a circuit member for a semiconductor device, which can offer such a semiconductor device, have been desired in the art.
Various CSPs (chip scale packages), which have realized a reduction in size of a semiconductor device, have been proposed. In this case, however, a resin and a film substrate, which have not been used in the art, should be used for packaging, posing a problem of reliability.
As described above, BGA using a double-sided wiring board shown in FIG. 39 has a complicate construction and suffers from many problems associated with the production and the reliability. Further, BGA provided with a circuit using a lead frame as a core material as shown in FIG. 40 has a problem of productivity and, in addition, involves a problem that it is difficult to maintain the positional accuracy of the inner lead and the quality.
On the other hand, a semiconductor device, which, as compared with TSOP and the like, is smaller in size and lower in cost, and a circuit member for a semiconductor device, which can offer such a semiconductor device, have been desired in the art.